The present invention relates to a structure for fastening a pad cover to a bag holder in a steering wheel having an airbag device.
For example, the description of Korean Patent Application Publication No. 10-2009-0026529 discloses such a fastening structure for a pad cover. This fastening structure will now be described with reference to FIGS. 15 to 17. A steering wheel with an airbag device 80 has a bag holder 70 and a pad cover 90 in addition to the airbag device 80.
The bag holder 70 is formed by pressing a metal plate, and is arranged below the airbag device. The airbag device 80 is fixed to the bag holder 70 with bolts 81 and nuts 82 (see FIG. 16). The bag holder 70 has a plurality of engagement holes 71, each of which is located in the vicinity of a part to which the airbag device 80 is attached, that is, near a bolt 81 and a nut 82. Each engagement hole 71 extends linearly along a substantially tangential direction of a circle the center of which coincides with the central axis CL of the bag holder 70, that is, along a substantially tangential direction of a circular opening 72. To distinguish the parts of the inner wall of each engagement hole 71, a pair of inner walls facing each other along the radial direction of the above circle, that is, the opening 72, are defined as “long inner wall surfaces 73”, and a pair of inner walls facing each other along the substantially tangential direction of the circle are referred to as “short inner wall surfaces 74.”
The pad cover 90 has a lid 91 covering the airbag from above, an annular accommodation wall 92 covering the airbag from the sides, and a plurality of engagement claws 93 at the lower end of the accommodation wall 92. The pad cover 90 is entirely formed of synthetic resin. Each engagement claw 93 is formed by an insertion portion 94 inserted into the engagement hole 71 and a claw portion 95 bulging from the lower end of the insertion portion 94 toward one of the long inner wall surfaces 73.
The insertion portion 94 of each engagement claw 93 is inserted into the corresponding engagement hole 71, and the claw portion 95 is hooked to the lower surface of the bag holder 70 in the vicinity of the adjacent long inner wall surface 73, so that the engagement claw 93 is locked. Through the locking of the claw portions 95 of the engagement claws 93, the pad cover 90 is fastened to the bag holder 70.
According to the above described fastening structure, the space D11 between the long inner wall surfaces 73 of each engagement hole 71 is set to be greater than the maximum thickness T11 of the engagement claw 93 as shown in FIG. 17, so that the engagement claw 93 can be inserted. The maximum thickness of the engagement claw 93 refers to the thickness of a part in which the claw portion 95 protrudes by the greatest amount.
When the airbag is inflated, the accommodation wall 92 and the engagement claws 93, which are made of synthetic resin, are pushed by the airbag and flexed to bulge outward. In contrast, the metal bag holder 70 is not deformed significantly when the airbag is inflated. The engagement holes 71 are not deformed significantly when the airbag is inflated, either. Thus, when the airbag is inflated, the engagement claws 93 are moved and flexed within the engagement holes 71. As a result, the claw portions 95 may be disengaged from the lower surface of the bag holder 70, and the engagement claws 93 may come off the engagement holes 71.
To deal with such problems, the apparatus of the above described publication has a pair of fastening pieces 96 in each engagement hole 71. Each fastening piece 96 has a support portion 97 parallel with the bag holder 70 and a coupling portion 98 for coupling the support portion 97 to the inner wall surface 74. With each engagement claw 93 inserted in the corresponding engagement hole 71, the corresponding fastening pieces 96 are bent at the boundary between the coupling portion 98 and the short inner wall surface 74. This causes a part of each support portion 97 to be inserted into the engagement hole 71 on the back side of the engagement claw 93, that is, on the side opposite to the claw portion 95. A back surface 94A of each insertion portion 94, which is on the side opposite to the claw portion 95, contacts the fastening pieces 96, so that the engagement claw 93 is prevented from moving in the corresponding engagement hole 71. As a result, the engagement claws 93 are prevented from coming off the engagement holes 71 without adding other components.
However, in the fastening structure for a pad cover according to the above described publication, the interstice between the insertion portion 94 of each engagement claw 93 and the corresponding long inner wall surface 73 cannot be easily eliminated when the fastening pieces 96 are bent and a part of each support portion 97 is inserted into the engagement hole 71. Whether it is possible to eliminate such an interstice is determined mainly by the width of the fastening pieces 96. If the width is less than an adequate value, the fastening pieces 96 cannot firmly press the engagement claws 93, and an interstice is created between each fastening piece 96 and the corresponding long inner wall surface 73. This causes the engagement claws 93 to chatter in the engagement holes 71. On the other hand, if the width is greater than the adequate value, the fastening pieces 96 cannot be easily inserted into the engagement holes 71.